Letter to the Editor

The VLT-FLAMES Tarantula Survey

Observational evidence for two distinct populations of massive runaway stars in 30 Doradus

¹ Institute of Astronomy, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
e-mail: hugues.sana@kuleuven.be
² German Aerospace Center (DLR), Institute for the Protection of Terrestrial Infrastructures, Rathausallee 12, 53757 Sankt Augustin, Germany
³ Department of Astronomy and Physics, Saint Mary’s University, 923 Robie Street, Halifax NS B3H 3C3, Canada
⁴ Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte, Natal 59072-970, Brazil
⁵ Anton Pannenkoek Astronomical Institute, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
⁶ Department of Physics & Astronomy, Hounsfield Road, University of Sheffield, Sheffield S3 7RH, UK
⁷ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ Argelander-Institut für Astronomie, der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heldelberg, Germany
¹⁰ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹¹ Max Planck Institut für Astrophysik, Karl-Schwarschild-Strasse 1, 85748 Garching, Germany
¹² Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
¹³ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
¹⁴ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, 08028 Barcelona, Spain
¹⁵ Centro de Astrobiología (-INTA). Campus ESAC, Camino bajo del Castillo s/n., 28 692 Villanueva de la Cañada, Madrid, Spain
¹⁶ Center for Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA Department of Physics, Columbia University, New York, NY 10027, USA
¹⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁸ Lennard-Jones Laboratories, Keele University, ST5 5BG Keele, UK
¹⁹ Armagh Observatory, College Hill, Armagh, BT61 9DG Northern Ireland, UK
²⁰ Royal Observatory of Belgium, Av. Circulaire 3, 1180 Uccle, Belgium
²¹ Programa de Pós-graduação em Física, Universidade do Estado do Rio Grande do Norte, Mossoró 59610-210, Brazil

Received: 4 August 2022
Accepted: 24 November 2022

Abstract

Context. The origin of massive runaway stars is an important unsolved problem in astrophysics. Two main scenarios have been proposed, namely: dynamical ejection or release from a binary at the first core collapse. However, their relative contribution remains heavily debated.

Aims. Taking advantage of two large spectroscopic campaigns towards massive stars in 30 Doradus, we aim to provide observational constraints on the properties of the O-type runaway population in the most massive active star-forming region in the Local Group.

Methods. We used radial velocity measurements of the O-type star populations in 30 Doradus obtained by the VLT-FLAMES Tarantula Survey and the Tarantula Massive Binary Monitoring to identify single and binary O-type runaways. Here, we discuss the rotational properties of the detected runaways and qualitatively compare the observations with expectations of ejection scenarios.

Results. We identified 23 single and one binary O-type runaway objects, most of them located outside the main star-forming regions in 30 Doradus. We find an overabundance of rapid rotators (v_e sin i > 200 km s⁻¹) among the runaway population, thus providing an explanation for the observed overabundance of rapidly rotating stars in the 30 Doradus field. Considerations of the projected rotation rates and runaway line-of-sight velocities reveal a conspicuous absence of rapidly rotating (v_e sin i > 210 km s⁻¹), fast-moving (v_los > 60 km s⁻¹) runaway stars in our sample, strongly suggesting the presence of two different populations of runaway stars: a population of rapidly spinning but slowly moving runaway stars and a population of fast-moving but slowly rotating ones. These are detected with a ratio close to 2:1 in our sample.

Conclusions. We argue that slowly moving but rapidly spinning runaway stars result from binary ejections, while rapidly moving but slowly spinning runaways could result from dynamical ejections. Given that detection biases will more strongly impact the slow-moving runaway population, our results suggest that the binary evolution scenario dominates the current massive runaway star population in 30 Doradus.